Spud Cannon with direct propane injection

This is my spud gun with a 2"x 36" barrel, a 1.5"X48" barrel, and a 2.5"X24" tennis ball barrel. I've installed direct propane injection and the ignition source is a bbq igniter. The chamber to barrel ratio is (C:B) 1.6:1 with the 1.5" barrel, 1.2:1 with the 2" and 1.1:1 with the tennis ball barrel. The chamber is made with 4" sch 40 pvc. All fitting and barrel pipe is also sch 40. The picture with the painted barrels and chamber reflects the specs above. The unpainted unit was version 1 and featured a larger combustion chamber, too large for my barrels.